1. Field of the Invention
The invention relates to a method of manufacturing a hollow blade for a turbomachine in which a plurality of primary sheet-like parts are assembled face to face and diffusion welded together in predetermined areas before being deformed to shape by pressurized gas and superplastic forming.
2. Discussion of the Background
The advantages of using large-chord blades in turbomachines are particularly apparent in the case of the fan rotor blades of bypass jet engines. Such blades must withstand severe operating conditions and, in particular, must have mechanical properties enabling them to withstand satisfactorily vibrations and impacts from foreign bodies. Also, the requirement for adequate blade tip speeds has stimulated efforts to reduce the weight of the blades, which aim can be achieved in particular by the use of hollow blades.
EP-A-0700738 describes a process for the manufacture of a hollow turbomachine blade, particularly a large-chord blade for a fan rotor, which process generally comprises the following steps:
a) defining the flat form of the primary sheet-like or substantially planar parts of the blade using CADCAM and digital simulation techniques; PA1 b) die-forging the primary parts of the blade in a press; PA1 c) machining the primary parts; PA1 d) depositing diffusion barriers on at least one of the primary parts according to a predefined pattern; PA1 e) assembling the primary parts and hot diffusion welding them together under isostatic pressure; PA1 f) pressurized gas inflation and superplastic forming of the welded assembly; and, PA1 g) final machining of the assembly. PA1 a) defining the flat form of the primary sheet-like parts of the blade by CADCAM and digital simulation techniques; PA1 b) die-forging the primary parts of the blade in a press; PA1 c) machining the primary parts; PA1 d) depositing diffusion barriers on at least one of the primary parts according to a predefined pattern; PA1 e) assembling the primary parts and hot diffusion welding them together under isostatic pressure; PA1 f) hot shaping the welded assembly by elongation of the fibres; PA1 g) hot forming a feed duct followed by unsticking of the primary parts in the regions coated with an anti-diffusion material; PA1 h) pressurized gas inflation and superplastic forming of the assembly; and, PA1 i) final machining of the assembly.
French Patent Application No. 9607329 describes such a process in which the assembly can be shaped by a twisting operation without risk of causing buckling undulations along the neutral fiber by carrying out a preliminary step of elongating the fibers distributed on either side of the neutral fibre.
French Patent No. 2739045 and French Patent Application No. 9610194 describe such a process in which, in view of the compaction of the primary parts in the regions of the diffusion barriers during the welding step, an operation to unstick the primary parts in these regions is carried out before performance of step (f). The unsticking step is achieved by hot forming a feed duct in a zone where the assembly has communicating passages between cavities. The cavities are therefore fed evenly and simultaneously, and the deformation rates can be controlled right from the start of the inflation cycle of step (f), thus ensuring the regularity and shape of the stiffeners. This results in a process with the following general operating sequence of steps:
In step (d) of this process it is known to deposit the anti-diffusion material by a silk-screen process. Embodiments are described in detail in French Patent Application No. 2739045. FIG. 1 illustrates a stage in the process when used to manufacture a hollow blade for a turbomachine fan from three primary parts defining an extrados skin 11, a central plate 12 and an intrados skin 13, at least two surfaces having been coated with an anti-diffusion material following a predefined pattern. In the stage shown in FIG. 2 the primary parts 11, 12, 13 have been assembled to form an assembly 14 using two centring pins 15, 16, and the assembly has been welded around its periphery in a neutral atmosphere in order not to contaminate the surfaces of the part. Before complete closure of the assembly by the peripheral welding, one or more tubes, such as 17 and 18, are added so as to communicate with the inner regions coated with the anti-diffusion material. At this stage the assembly is ready for diffusion welding which, in the case of a TA6V or TAD4E type titanium alloy, is performed at a temperature above 880.degree. C. However, as described in the aforesaid French Patent Application NO. 2739045, it is often necessary to carry out a preheating, treatment of the anti-diffusion material before the diffusion-welding step because the organic binders which are included in anti-diffusion materials, and which are needed to carry the material and ensure mechanical retention of the deposit on the coated surfaces, must be removed in order to ensure intimate contact between the surfaces and in order not to pollute the contacting surfaces when the temperature rises to above 500.degree. C. for diffusion welding. The binders are completely eliminated by thermal breakdown between 200 and 400.degree. C. and the gases evolved are removed by a flow of an inert gas, for example argon, or by pumping. During this operation the particles of the anti-diffusion material become very mobile since the mechanical bonds provided by the binder are completely broken down, and the assembly therefore requires very careful handling to ensure that there is no migration of particles to the uncoated areas.